1. Field of the Invention
The field of the invention is that of touch-sensitive surfaces or “touchscreens” with capacitive detection and, more particularly, so-called “multitouch” touch-sensitive surfaces allowing for the detection of two simultaneous touches. This function is essential for producing, for example, image “zooms” or rotations. This invention can be applied to different uses but it is particularly well suited to the constraints of the aeronautical domain and aircraft instrument panels.
2. Description of the Prior Art
The so-called “projected” capacitive detection consists in producing a detection matrix arranged so as to detect the local variations of capacitance introduced by the proximity of the fingers of the user or of any other conductive designating object. The so-called projected capacitive technology has two main variants which are:                “self capacitive” detection which consists in reading the rows and then the columns of the array of keys of the matrix;        so-called “mutual capacitive” detection consisting in reading each intersection of the array of keys of the matrix.        
The “mutual capacitive” technology entails reading the entire pad. Thus, if the matrix has N rows and M columns, N×M acquisitions must be made, which makes the production of pads of large size, with high resolution and low response time, problematical. Furthermore, the capacitance to be measured in “mutual capacitance” mode is lower than that obtained in “self capacitance” mode, which makes the use of gloves by the user problematical.
The advantage of the “self capacitive” detection is that, for the above pad, the system requires only N+M acquisitions to produce a reading of the matrix. The major drawback of the “self capacitive” technology is the absence of any simple solution for resolving the problem of phantom touches or “ghosts”. FIG. 1 illustrates this problem. It represents a partial view of a matrix MLC of conductive rows and columns. In this figure and in the subsequent figures, the touches are represented by two fingers. When two simultaneous touches occur at (XI, YI) and (XJ, YJ), the system detects the two columns (XI, XJ) and the two rows (YI, YJ) that have been stressed. These two rows and these two columns correspond to real touches but also to two phantom touches G positioned at (XI, YJ) and (XJ, YI) without the system being able to decide a priori which are the correct touches.
To resolve this problem of ghosts, countermeasures have been developed. A first solution consists in producing a time discrimination based on the low probability of the occurrence of a simultaneous touch given that the acquisition speed of the pad is very rapid. However, perfectly simultaneous touches corresponding, for example, to the approach of two fingers to perform an image rotation on the touch-sensitive surface, are not processed. The term “multitouch” device cannot therefore be truly applied.
The patent application US2010/0177059 proposes a method with which to eliminate the doubt by measuring row/column coupling capacitances in the vicinity of the touch. Since this coupling does not exist in the case of a ghost, it is then easy to determine the position of the real touch. However, such a measurement requires a large number of analog switches, seven for each row/column pair in the case cited. In addition to the not inconsiderable excess cost, the addition of switches increases the coupling capacitance relative to the ground when they switch over the rows or the columns. In fact, they act as if a large number of fingers were placed on the pad.
As an example, an excellent switch in JFET (Junction Field Effect Transistor) technology has a coupling capacitance of 10 pF, whereas a finger has a capacitance of the order of 1 pF. If 4 switches are implemented, a variation of 1 pF has to be measured on a value of 40 pF instead of 10 pF with a single switch. The result of this is a loss of sensitivity in a ratio of 4. This loss of sensitivity is problematical when the user is wearing a glove or when, in the presence of electromagnetic noise, the signal/noise ratio is degraded.
The capacitance measurement poses other problems. There are primarily three methods for measuring a capacitance.
The oldest is the capacitive divider bridge, one arm of which consists of a reference capacitance and the other of the capacitance to be measured. A second method uses a relaxation oscillator whose frequency depends on the value of the capacitance to be measured. These two methods are known to be sensitive to reading noise, in particular in cases of radiofrequency interference, which are commonplace on an aircraft.
The third method, and the most widely used these days on consumer products for capacitive protection on touch pads, is measurement by charge transfer. There are different variants which give a good signal-to-noise ratio. This method consists in powering the capacitance to be measured by a “burst” of square pulses until the latter is charged to a reference value. The number of pulses needed to obtain the reference charge is representative of the capacitance to be measured.
However, this measurement requires a plurality of switches, necessary to the transfer of the charges and to the generation of the bursts. The stray capacitances of these switches limit the dynamics of the signal and degrade the signal-to-noise ratio.
Another drawback with this method is its weak robustness to the electronic interferences emitted at a frequency roughly equal to that of the bursts. In practice, the rows and the columns then behave as antennas and pick up the electromagnetic waves. They will induce stray electrical currents in addition to the measurement bursts and provoke erroneous measurements.
Finally, the capacitive measurement methods mainly use an alternating signal of relatively high frequency to perform the measurement. In certain environments such as aircraft cockpits, the electromagnetic emission levels of the equipment have to be considerably low in order not to disturb certain sensitive equipment such as the sensors or antennas. The charge transfer measurement method implements square signals. These signals generate harmonics which create disturbances over a wide frequency band, well above the thresholds allowed by the standards.
In conclusion, the projected capacitive touchscreens currently have many drawbacks which make them difficult to use in an aircraft cockpit or in any critical environment. In practice, as has been seen, the detection of the touches may be falsified by phantom touches or by external electromagnetic interferences. Furthermore, the measurement principle may interrupt the electromagnetic environment.